Maybe not family computers so much any more, but shared devices, absolutely.

It still needs to be per user, per app. If I hand my phone to my kid or my niece, I don't need them looking up or accidentally seeing adult content because the apps don't ask anymore.

If the apps stop asking the user, and instead just query the OS, there's no longer any connection to the current user. So porn sites, for example, or any app that might have adult-only content, would still have to ask. In which case - what's the point of the OS age requirement? This is in no way more secure than the apps or websites just asking the user.

What about servers? I have several devices in my house that serve content to the rest of the house or provide other services. Would they need my birthday, even though my kids use the services? What about gaming consoles, or TVs? IoT devices? Does my thermostat, garage door opener, living room lamp, or washing machine need my birthdate?

This whole thing is truly a slippery slope that hasn't been thought through, at least on its face. Unless, of course, the whole point of this legislation has nothing to do with "protecting children".